This disclosure relates very generally to a deep bed filter having one or more media layers that are backwashed substantially in place by reverse liquid flow.
Deep bed filters have evolved through the years and have become more efficient and capable of handling variable filter operating conditions. For example, probably one of the most widely used filters through the years has been the mono-bed filter comprised of sand. Sand, however, has its drawbacks because it consists of a distribution of different grain sizes, even when sieved to within a fairly restricted mesh size range. This grain size distribution results in a classification or stratification of granules during backwash. More specifically, in the backwashing step the filter bed is expanded and the lighter smaller materials tend to rise to the top and the heavier larger materials tend to gravitate to the bottom. Therefore, materials are classified with the finest filter particles in the top of the filter and the coarsest filter particles in the bottom.
One of the most ideal ways for a deep bed filter to operate is to have the influent go through the coarsest layer first and then through progressively finer filter media layers. However, in the normal down-flow, mono-bed filter the least ideal condition exists as the flow is from the finest to the coarsest. This often creates a condition that the incoming contaminant cannot penetrate the inlet surface and a plugging, or surface loading, condition occurs. This creates a fast pressure rise and puts the filter out of condition in an abnormally short period of time, requiring premature backwashing.
The next step in the improvement of the mono-bed filter was the advent of the dual media filters. These filters typically consist primarily of a top layer of anthracite and a bottom layer of sand, the anthracite being lighter and larger than the sand, and the sand consisting generally of the same size particles as used in the mono-bed. The dual media filters of this type provide at least two advantages over the mono-media filter. First, the anthracite presents a more open and porous surface to the influent, thus allowing coarser particulates to penetrate the surface. Second, a mixing at the interface between the two medias results in a more porous strata at the top of the layer of sand, again allowing penetration into the sand bed. This arrangement, even though both media tend to classify, is far superior because of the larger, lighter anthracite on the surface and is now the most accepted dual media filter in the art. Even though there is far less surface plugging tendency in this arrangement than in the mono-bed, high concentration of dirt and lint like materials still surface load and plug this type of filter fairly easily.
The basic dual media filter has been modified in various respects, for example by adding a material with a greater specific gravity and finer grain size than the sand, as shown in U.S. Pat. No. 3,343,680. The finer, heavier material represents the bottom media, the second being sand and the top anthracite. The size of the top two materials is similar to that used in the standard dual-media concept. This concept allows somewhat finer filtration and minimizes breakthrough; however, it is still subject to the problem of surface load plugging, as discussed above.
Another concept is shown in Applicant's earlier U.S. Pat. No. 3,695,433, where a large organic polymeric material in the size range of 2.5 to 6.5 millimeters is disclosed for a mono-bed. This arrangement was successful in removing heavy loads and lint like materials, however, was somewhat lacking in a high enough percentage of removal of fine materials. Therefore, in some environments, this filter had to be followed up with an additional polishing filter such as a dual-media filter. Two distinct filters were required because the plastic beads had to be backwashed by recirculation through external means and therefore would not fit in with the normal flow backwash technique used in standard filters.
Further modification of this principle is shown in Applicant's U.S. Pat. No. 3,814,425 in which large organic polymeric materials were used on top of a layer of sand. This was effective in filtration and did not plug up from concentrated loads and lint like materials. However, backwashing was difficult because the sand had to be backwashed in situ and the organic particles had to be recirculated and backwashed outside of the filter. This created a problem of balancing a flow for the proper quantity of water to expand the sand and this quantity had to be removed at the same time by external scrubbers. The flow equilibrium, although not impossible, was difficult to achieve as changes of temperature had a pronounced effect on the overall results.
There have been other efforts to overcome heavy dirt loads and plugging materials coming to deep bed polishing filters. However, most of these techniques have depended upon gravity settling and various techniques of coagulation to reduce the dirt load. Most of the time this condition is impossible to control. Such is the case with a sanitary system where the equilibrium is upset when a storm or an abnormal condition occurs or when a toxic material is accidentally dumped into a sanitary system. When a condition of this nature occurs, the clarifiers, which are in a delicate equilibrium, are completely upset and the sludge blanket tends to overflow to the filters. The normal practice is to immediately divert this flow away from the filters and most of the time this high concentration of contaminants is dumped into the streams. The inadequacy of the current filters therefore is quite evident in occurrences of this nature.